read or readall methods no longer lose data when an underlying read
system call is interrupted within an io module object.  IOError is no
longer raised due to a read system call returning EINTR from within
these methods.

This is a backport of changeset 781b95159954 from 3.2.
The earlier 2.7 changeset 67dc99a989cd already fixed this for the
builtin python 2.x file object.